1. Field of the Invention
The present invention relates in general to power control apparatuses and, in particular, to a power control system for measuring the current drawn by a power amplifier which is indicative of its power output and altering the input to the power amplifier so as to alter its power output.
2. Background Art
Power amplifiers are used in radio transmitters to produce a signal with sufficient power for transmitting over a distance. Most, if not all, radio transmitters measure the power output of this amplifier, toward being able to adjust the output of this amplifier to match a desired value. The most widely used approach to accomplish this measurement utilizes a directional coupler on the post power amplifier transmission line which is connected to a diode detector. Overall, this approach is expensive to implement, both financially and in terms of space requirements. This is particularly true in high frequency radio applications where Schottky diodes--which are rather expensive--would be necessary to provide an accurate measurement of minor variations in power amplifier output.
Current mirrors have long been utilized in electrical circuits as a current source device. Particularly in integrated circuits, multiple current mirrors are used to "steer" an externally generated current in various multiples of the known generated current to various portions of the circuit. These uses of the current mirror all require a constant current source to supply the mirror with a known programming current, which, in turn, sets the various currents.
Another approach to power output measurement measures the operating current drawn by the power amplifier. The operating current drawn by the power amplifier is related to the amplifier output power, or gain, such that measuring the current drawn by the power amplifier may be used to determine its output power. This approach is valid for class B, C or BC power amplifiers. These power amplifier classes draw an operating current proportional to their power output. The measurement of this current can be accomplished in various conventional ways, such as inserting a resistor in the source line and measuring the voltage drop across the resistor using an operational amplifier; an expensive approach. The introduction of a resistor into this line diminishes the power received by the power amplifier. Of course, the value of the resistor can be minimized to minimize power dissipation, however, this would be at the expense of requiring a more sensitive operational amplifier, which is increasingly expensive given increased sensitivity.
It is thus an object of the present invention to provide a low-cost approach to measuring the power output of a power amplifier.
It is a further object to provide a measuring approach which introduces a minimal voltage drop into the connection between the power source and the amplifier.
These and other objects will become apparent in light of the attached specification and drawings.